Rayleigh fading is a phenomenon associated with radio communications where multiple reflections of a transmitted signal constructively and destructively superpose at points in space according to the relative phase and amplitude of all the reflected signals. This spatial interaction of multi-path signals produces both strong peaks (local maximums) and deep nulls (local minimums) of relative signal strength with regard to varying position. Often the fading can result in signal “holes” where the signal strength will drop below a minimum threshold of reception of a radio receiver, e.g., a mobile radio station or terminal. Depending on the relative phase of the interacting signals, the peaks and nulls of signal strength are generally separated by a fraction of the free space wavelength of the carrier frequency of the transmitted signal. These signal strength fluctuations present a significant and challenging problem, particularly in the mobile radio communications environment.
One mechanism which addresses the problems of Rayleigh fading is a space diversity technique where two or more antennas are separated by a minimum of one half of a wavelength of carrier frequencies to be received. At radio frequencies, for example, a deep signal fade can be overcome by moving the antenna only a few inches. As a result of separating two antennas by an appropriate distance, each antenna receives a signal whose fading pattern is uncorrelated with the fading pattern of the signal received by the other antenna. Improved signal reception is then achieved simply by programming the receiver to select (using one or more conventional techniques) the antenna with the strongest signal. More sophisticated techniques combine the signals from both antennas. Commonly-assigned U.S. Pat. No. 5,392,054 to Bottomley describes an antenna assembly for diversity reception in a mobile radio terminal.
Transmit diversity antennas have also been considered to enhance performance including system capacity and data throughput. With transmit diversity, the required transmit signal power to provide a particular data rate or a particular signal quality can be reduced. Transmit diversity involves transmitting data on two or more antennas, where the geographical separation between the antennas leads to path loss characteristics that are independent from antenna to antenna. However, the signal transmitted from the multiple antennas can combine constructively or destructively over the air at a receiving station, and the receiving station cannot coherently combine signals from the transmit diversity antennas. To alleviate this problem, different signals representing the same data may be transmitted from the two transmit antennas.
One approach for diversity transmission for mobile terminals might be to switch between two or more transmit antennas (“antenna hopping”) on a fixed schedule. Such antenna hopping avoids the likelihood of bad fades in a statistical fashion, i.e., the probability of a bad fade affecting both (all) of the spatially-separated antennas is low. But lacking knowledge of the uplink transmission channel conditions, this kind of fixed schedule antenna hopping has limited promise. In other words, the selected antenna may not be the best antenna for the current channel conditions.
In another approach referred to as delay diversity, the signal transmitted from a second antenna is delayed with respect to the signal transmitted from a first antenna. At the receiver, the resulting signal appears as if it passed through a channel with dispersion, and a well-designed equalizer may be used to achieve performance gain over transmission from a single antenna. Yet another approach uses space-time coding, where the signal is encoded across two transmit antennas in such a way as to provide diversity gain at a receiver.
Using two or more antennas to transmit from the mobile station on the uplink to the base station to improve performance introduces additional cost and power concerns. In general, each transmit antenna requires a power amplifier along with other transmit diversity branch circuitry. This is particularly true for the delay diversity and space-time coding approaches described above. Not only are power amplifiers costly electronic components, they also consume considerable battery power. The antenna hopping approach described above can use a single power amplifier, but its performance is limited. Hence, it would be desirable to provide a transmit diversity arrangement that does not increase the number of power amplifiers in a mobile station and that also provides significant performance benefits.